This invention relates to self-phasing electromagnetic energy arrays and more particularly concerns a coherent optical adaptive system and method of operation.
Self-phasing laser arrays, broadly described as coherent optical adaptive systems, receive their phasing information from the target itself and focus a relatively narrow beam upon a target that may be readily tracked by the array. In addition to the small width of the focused beam, significant advantages of coherent optical adaptive techniques include compensation for perturbations induced by both the beam propagating medium and factors within the transmission system itself. In these adaptive systems a multi-element or multi-aperture transmitting and receiving array is employed to sample the relative phasing or phase distribution of a wave received from a reflecting target. The phases of received signals are measured and an adapted wave is transmitted from the array elements having a phase that is adjusted in accordance with the measured phase so as to compensate for perturbations and focus an energy lobe on the target. In effect, the adaptive phased array receives a wave reflected from the target and which is distorted in shape because of the intervening turbulence, blooming, vibrations and other disturbances in the propagating medium. The system transmits a wave that is predistorted in such a way that it will be focused on a reflecting target after again passing through the intervening propagating medium. The predistortion, or correction, of the transmitted wave to compensate for intervening perturbations varies with time since the perturbations themselves vary with time, and thus the system is termed an adaptive phased array.
Among several different arrangements for implementing an adaptive phased array are the multidither systems and the phase conjugation systems.
A multidither system is described in "Coherent Optical Adaptive Techniques" by Bridges, Brunner, Lazzara, Nussmeier, O'Meara, Sanguinet and Brown, Jr. in Applied Optics, Vol. 13, No. 2, February, 1974, pages 291-300. In the multidither system a time-varying phase disturbance is introduced to move the fringe pattern transversely and a servo loop is employed to control a transmitter phase shifter to position the transmitted beam symmetrically upon the reflecting target.
In phase conjugation adaptive phased arrays, the phase distribution of the wave reflected from the target as received at the several elements or apertures of the array is measured by an array of heterodyne receivers employing a local oscillator as a phase reference. The signals transmitted from each element of the array are then phase adjusted so as to have a phase which is the phase conjugate of the phase of the signal received at respective elements. This phase conjugation of the signal transmitted from each of the array elements produces a diffraction-limited beam focused on the reflecting target and compensated for perturbations and disturbances, both in the propagating medium and within the system. A phase conjugation adaptive system of this type is disclosed in a copending application of Cecil L. Hayes and Walter C. Davis, Ser. No. 398,282, filed Sept. 17, 1973, entitled "Self-Compensating Interferometer". This application of Cecil L. Hayes et al is a continuation-in-part of application Ser. No. 282,621, filed Aug. 21, 1972 by Cecil L. Hayes et al and entitled "Self-Compensating Interferometer". Both of these patent applications of Hayes et al are assigned to the assignee of the present invention and the disclosures of both of these applications are incorporated herein by this reference as though fully set forth.
Adaptive phasing techniques have important applications in the fields of optical radar, communications and high-energy beam power delivery to remote targets. In many of these applications it is desirable to offset the focused beam with respect to a bright reflecting target. Thus, the receiving station of a communication system may employ a bright reflector which is offset from the receiver itself so as to aid in locating and tracking of the receiving station by the transmitting station. In other applications it may be desirable to scan a target for identification or other purposes while the beam is locked on and continues to track the moving target. Further, it may be desired to offset the beam from the target because the energy of the beam may cause undesired changes in the glint (reflecting) point and change or considerably decrease its reflective qualities. Accordingly, several types of offset pointing techniques have been considered for adaptive phased arrays. These may be grouped as those that operate full time in the adaptive mode and those that operate only part time in the adaptive mode. In both of these groups, in phase conjugation systems, the received signal is compared with a reference that has been phase shifted by applying a phase slope bias across the array from aperture to aperture. The phase slope bias is combined with the reference with respect to which the phase of received signals is measured. It produces a diffraction-limited lobe of the transmitted beam that is angularly offset from a line between the array and the reflecting target. The real or reflecting target is defined herein as a glint or point on an object that reflects impinging energy and is distinguished from a simulated, fictitious or offset target that is a location on an object, which location may produce little or no reflection of impinging energy. In the time-shared method of offset pointing, the normal phase conjugation process, that is, the basic adaptive technique without phase slope bias, is time shared with the offset pointing technique and the two steps are repeated cyclically. Thus, during the normal phase conjugation step of the two-step cycle, the transmitter phases are adapted and will retain focus of the beam upon the reflecting target. On the other hand, during the offset part of the cycle, the transmitter phases are not adapted and accordingly, the beam may defocus or wander due to time-varying system and atmospheric disturbing parameters.
In continuously adapted methods, phase conjugation is applied to the return from the offset beam. However, in general a smaller or weaker signal is available when the beam is offset because the main diffraction lobe remains incident on the offset target and the latter is assumed to have a small or negligible reflectivity. The return from the offset beam (illuminating both reflecting and offset targets) in this continuously adapted offset pointing technique is conjugated and retransmitted each time with the same phase slope bias and thus locks the offset beam to the glint point or reflecting target. However, since the adaptive phase conjugation of the transmitted beam is based upon a signal received from the reflecting target, this continuous offset pointing technique may exhibit severe instability. If a relatively large amount of energy is reflected from the offset target, as compared with the energy reflected from the reflecting target (the target upon which the adapted beam is desired to be locked), the normal adaption process may be disturbed and the beam may wander to lock either on an adjoining less reflective target or upon the offset target itself. Thus, the time-shared offset pointing may readily defocus or wander and the continuous offset pointing is relatively unstable. Accordingly, it is an object of the present invention to provide a continuously adaptive track and offset operation in an adaptive phased array that avoids or minimizes these disadvantages.